Name

zmq_pgm - ØMQ reliable multicast transport using PGM

Synopsis

PGM (Pragmatic General Multicast) is a protocol for reliable multicast transport of data over IP networks.

Description

ØMQ implements two variants of PGM, the standard protocol where PGM datagrams are layered directly on top of IP datagrams as defined by RFC 3208 (the pgm transport) and "Encapsulated PGM" or EPGM where PGM datagrams are encapsulated inside UDP datagrams (the epgm transport).

The pgm and epgm transports can only be used with the ZMQ_PUB and ZMQ_SUB socket types.

Further, PGM sockets are rate limited by default. For details, refer to the ZMQ_RATE, and ZMQ_RECOVERY_IVL options documented in zmq_setsockopt(3).

The pgm transport implementation requires access to raw IP sockets. Additional privileges may be required on some operating systems for this operation. Applications not requiring direct interoperability with other PGM implementations are encouraged to use the epgm transport instead which does not require any special privileges.

Addressing

A ØMQ endpoint is a string consisting of a transport:// followed by an address. The transport specifies the underlying protocol to use. The address specifies the transport-specific address to connect to.

For the PGM transport, the transport is pgm, and for the EPGM protocol the transport is epgm. The meaning of the address part is defined below.

Connecting a socket

When connecting a socket to a peer address using zmq_connect() with the pgm or epgm transport, the endpoint shall be interpreted as an interface followed by a semicolon, followed by a multicast address, followed by a colon and a port number.

An interface may be specified by either of the following:

The interface name as defined by the operating system.

The primary IPv4 address assigned to the interface, in its numeric representation.

Interface names are not standardised in any way and should be assumed to be arbitrary and platform dependent. On Win32 platforms no short interface names exist, thus only the primary IPv4 address may be used to specify an interface. The interface part can be omitted, in that case the default one will be selected.

A multicast address is specified by an IPv4 multicast address in its numeric representation.

Wire format

Consecutive PGM datagrams are interpreted by ØMQ as a single continuous stream of data where ØMQ messages are not necessarily aligned with PGM datagram boundaries and a single ØMQ message may span several PGM datagrams. This stream of data consists of ØMQ messages encapsulated in frames as described in zmq_tcp(7).

Pgm datagram payload

The following ABNF grammar represents the payload of a single PGM datagram as used by ØMQ:

datagram = (offset data)
offset = 2OCTET
data = *OCTET

In order for late joining consumers to be able to identify message boundaries, each PGM datagram payload starts with a 16-bit unsigned integer in network byte order specifying either the offset of the first message frame in the datagram or containing the value 0xFFFF if the datagram contains solely an intermediate part of a larger message.

Note that offset specifies where the first message begins rather than the first message part. Thus, if there are trailing message parts at the beginning of the packet the offset ignores them and points to first initial message part in the packet.

The following diagram illustrates the layout of a single PGM datagram payload: